The present invention relates generally to mitigating compressor surge conditions in a vehicle fuel cell power system, and more particularly to systems and methods for mitigating compressor surge conditions based on the speed of the compressor.
Fuel cells are one alternative to using gasoline or related petroleum-based sources as the primary source of energy in vehicular propulsion systems. In particular, by combining reactants in an electrochemical reaction within the fuel cell, electric current can be generated and used to power a motor or perform other useful work. In one form, the motor being powered by the electric current may propel the vehicle, either alone or in conjunction with a petroleum-based combustion engine. In automotive applications, individual fuel cells may be arranged in series or parallel as a fuel cell stack in order to produce a higher voltage or current yield. Furthermore, still higher yields may be achieved by combining more than one stack.
In a typical fuel cell, hydrogen or another reactant gas is supplied to the anode of the fuel cell, while an oxygen-based reactant (for example, ambient air) is supplied to the cathode of the fuel cell. The hydrogen is catalytically broken into electrons and positively charged ions such that an electrolyte layer that separates the anode from the cathode allows the ions to pass to the cathode while preventing electrons from doing the same. Instead, electrons are routed around the electrolyte layer through a load and back to the cathode, allowing electrical power to be harnessed. At the cathode, the ions, electrons, and supplied oxygen or air are typically combined to produce water and heat. Individual fuel cells may be arranged in series or parallel as a fuel cell stack in order to produce a higher voltage or current yield. Furthermore, still higher yields may be achieved by combining more than one stack.
To improve the delivery of the reactant gases, pressurized sources are often used. For example, the air being delivered to the cathode side of a fuel cell system is often by way of a compressor, where ancillary equipment—such as valves, controllers or the like—is used to regulate the airflow between the compressor and fuel cell. An inherent attribute of a compressor-aided delivery system (at least as it relates to cathode-side operation) is that the cathode's pressure and flow control are coupled together; this coupling means that stable operation can often be best achieved through a feedforward-based control strategy to take advantage of known or ascertainable mathematical relationships. In this way, a command signal based on known operational characteristics of the compressor may be sent to the compressor to affect a change therein in a way that will ensure predictable, repeatable response.
Despite the advantages of feedforward-based control strategies for compressors, certain operating conditions may jeopardize system operability and component durability. As such, the use of compressors within a vehicular fuel cell system based on such strategies remains a challenge, especially as it relates to operating conditions that can lead to surge or related undesirable compressor phenomena. When a surge is present, the back pressure from the compressor's outlet is sufficiently high to prevent the compressor from pumping as designed. This causes the gas flow in the compressor to reverse direction, potentially leading to damage to the thrust bearings, blades and other upstream compressor components.